A handy piece of equipment for any cyclist with a rucksack or panniers is this cheap and yet very sturdy dry liner from Halfords, currently retailing for £2.99. I think I paid £5,99 back in 2008, so a big drop in price.
I have had mine for three years now and I use them every working day to carry food and my clothes inside my rucksack. Whilst my rucksack has a waterproof cover, the convenience of simply taking these bags out (I have two) when I get to work is simply worth it. I have a shower at work too, so the one with my clothes in, I simply carry over to the shower room. No hassle. If it gets wet, no biggy, they are designed to get wet.
You don’t have to use them just for cycling either, they are great if you go camping and want to ensure your clothes stay dry in your kit bag, or if you want anything to stay dry, such as electrics, food or anything else you can think of.
For it’s sturdiness, 99% waterproof (not tried it submerged) and for its price, this gets a 5 bike rating from me.
Update 11th January 2017
I still have two of these bags and they are still going strong. They have been on many trips with me and I have even used it as a seat to protect me from the wet grass. The best buy ever.